dt-bindings: fsl: scu: add thermal binding

NXP i.MX8QXP is an ARMv8 SoC with a Cortex-M4 core inside as
system controller, the system controller is in charge of system
power, clock and thermal sensors etc. management, Linux kernel
has to communicate with system controller via MU (message unit)
IPC to get temperature from thermal sensors, this patch adds
binding doc for i.MX system controller thermal driver.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1582330132-13461-1-git-send-email-Anson.Huang@nxp.com